BMI Calculator for Women, Men & Teens

What is BMI (Body Mass Index)?

Body Mass Index (BMI) is a widely accepted measure used to estimate body fat based on your height and weight. It was first introduced by Adolphe Quetelet in the 19th century and remains a fast screening tool to assess general weight status.

The calculation is straightforward:

Using the metric system: divide your weight in kilograms by the square of your height in meters.
Using the imperial system: divide your weight in pounds by your height in inches squared, then multiply the result by 703.

According to standard guidelines, BMI categories for adults are:

Underweight: BMI less than 18.5
Normal (healthy) weight: BMI between 18.5 and 24.9
Overweight: BMI between 25.0 and 29.9
Obesity Class 1: BMI between 30.0 and 34.9
Obesity Class 2: BMI between 35.0 and 39.9
Obesity Class 3 (severe obesity): BMI of 40 or more

Remember, BMI is not a direct measure of body fat but is a simple tool to help identify potential weight problems.

BMI Formula

The Body Mass Index (BMI) is calculated using a person’s weight and height. The standard formula depends on the measurement units you use. In the metric system, BMI is calculated by dividing your weight in kilograms by your height in meters squared. If you’re using centimeters, you’ll need to adjust the formula slightly. For those using pounds and inches, the imperial formula includes a conversion factor. Here are both formulas:

Metric (kg/m²):
BMI = weight (kg) / [height (m)]²
Metric with centimeters:
BMI = (weight (kg) × 10,000) / [height (cm)]²
Imperial (lbs/in²):
BMI = (weight (lb) / [height (in)]²) × 703

These formulas give a quick estimate of body fat levels and help categorize individuals into weight ranges like underweight, normal, overweight, or obese.

How BMI Differs by Age and Gender

BMI for Women and Men

While the general BMI categories are the same for both men and women, women tend to have a naturally higher percentage of body fat. This means women may reach a higher BMI than men without having excess body fat. People searching for BMI calculator for women or BMI calculator for men often want to understand these subtle differences, though the cutoffs remain universal.

BMI for Teens

BMI for teens (ages 2–19) works differently. Instead of fixed categories, it uses percentiles:

Healthy weight: between the 5th and 85th percentile for age and gender
Overweight: between the 85th and 95th percentile
Obese: greater than the 95th percentile
For teens, BMI must be interpreted according to age and gender growth charts.

BMI by Age

As people age, muscle mass typically decreases, and fat tends to increase. Some studies suggest that older adults may have slightly higher BMI but still be metabolically healthy. Still, the standard BMI cutoffs apply for simplicity.

BMI in kg and lbs

You can calculate BMI using either:

Kilograms and meters (kg/m²) — common internationally
Pounds and inches (lbs/in²) — common in the United States
Using a BMI calculator in kg or lbs allows flexibility for global users.

Example of BMI Calculation

Let’s go through a simple example.

Anna is a 30-year-old woman who weighs 70 kilograms and is 1.68 meters tall.

Step 1: Square the height (1.68 × 1.68 = 2.8224).
Step 2: Divide the weight by the squared height (70 ÷ 2.8224 ≈ 24.8).

Anna’s BMI is 24.8, which places her in the healthy weight category but right at the upper limit.

If you prefer pounds and inches:

Anna weighs around 154 pounds and is 66 inches tall.
Step 1: Square the height (66 × 66 = 4356).
Step 2: Multiply weight by 703 (154 × 703 = 108,262).
Step 3: Divide (108,262 ÷ 4356 ≈ 24.8).

Interpreting Your BMI Result

A BMI under 18.5 is considered underweight and may indicate nutritional deficiencies.
A BMI between 18.5 and 24.9 is considered healthy, associated with a lower risk of heart disease and diabetes.
A BMI between 25 and 29.9 is considered overweight and may signal an increased risk of chronic conditions.
A BMI of 30 or more is classified as obesity, with rising health risks as the number increases.

BMI is a useful general guide but has limitations:

It doesn’t distinguish between muscle and fat (e.g., athletes may have high BMI but low fat).
It doesn’t account for fat distribution (e.g., belly fat).
It’s not adjusted for certain life stages like pregnancy or aging.

Tips to Maintain a Healthy BMI

Choose balanced meals rich in fiber, protein, and healthy fats.

Limit processed foods and sugary drinks.

Exercise regularly, aiming for at least 150 minutes per week.

Monitor your weight monthly, especially during lifestyle changes.

Consult healthcare professionals for personalized advice.

Consider additional measurements like waist circumference for a better understanding of your health.

Frequently Asked Questions

What is a healthy BMI for women?

A healthy BMI for adult women is between 18.5 and 24.9. Some experts suggest women naturally fall toward the upper end of this range due to body composition differences.

How do you calculate BMI for men?

BMI is calculated the same way for men and women using the standard formula: weight divided by height squared. Categories remain identical.

Is BMI different for teens?

Yes, for teens BMI is evaluated using age- and gender-specific percentiles, not fixed numbers.

How does age affect BMI?

As you age, your metabolism slows, and muscle mass decreases, so BMI may underestimate body fat in older adults.

Can I calculate BMI in pounds?

Yes, use the imperial formula: (weight in pounds ÷ height in inches squared) × 703.

Is BMI reliable for athletes?

Not always. Athletes may have high BMI due to muscle mass, but low actual body fat.

Should I rely only on BMI?

No, BMI is a quick screening tool but should be combined with other health markers like waist size, cholesterol levels, and blood pressure.